Another trick I use with clouds.  Use rock latices, never spheres or other primatives.  The rough edges allow for more interesting shapes.  I like to create a pile of them, then use the 3d scatter to change their size, location and orientation, cull out the smallest ones that interfere with the scene's flow, move some that don't look right, and light it accordingly.  Works really well for those big thunderheads.

Of course, in post work with photoshop, if you have sharp edges on a cloud that look fake, use the smudge tool at say 30% and smear the edges of the clouds till it looks fluffy.

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